Paul's missionary life was full of apparently impossible challenges - but met by supernatural strength and wisdom.  The preceding verses (Philippians 4:4-12) show how Paul was always praising the Lord, always praying and always receiving God's peace.  Even in hardship he was content, and in prosperity he was not proud. He knew his limitations; but was confident of God's power.  He did not set a personal 'energy budget' telling God what he could and could not do; he was certain that he could do whatever God asked of him.  His confidence was not in his own ability or power; but in his weakness he knew that God would give strength to him.

Perhaps Paul sets the theological scene in Galatians 2:20, “I have been crucified with Christ and I no longer live, but Christ lives in me. The life I now live in the body, I live by faith in the Son of God, who loved me and gave himself for me.”  Dead people cannot do much!  That is how Paul describes himself - he identified with Jesus Christ in His death, and also in His resurrection life.  The power comes from God who lives within the person who has welcomed Him and His life.

Don't forget that Paul was in prison awaiting trial.  Prior to receiving the gifts from Philippi, he was sustained by the Lord. Now he had more than enough to meet his needs (Philippians 4:18).  But he wanted the Macedonian church to know that although he was very glad that they had sent provisions and a helper, his strength still came from God (Philippians 4:11-12).  That kind of quiet inner confidence about the present and the future is the work of God's indwelling Holy Spirit who longs to guide everybody who trusts in Jesus.  It is not proportional to how we feel, or our resources nor even the encouragement from fellow-believers: it is God's gift to people who recognise their weakness (2 Corinthians 12:9-10), and yet are determined to obey His next command in His strength.

This verse applies to obeying God in everyday things, as well as special acts of ministry, and for dealing with every circumstance which God allows.  It does not apply to the fantasy 'wish-list' in which a few immature believers think that God is obliged to pass their exams for them or get them promotion at work.  The power of Christ is for the work of Christ so that the glory will go to Christ.  His gifts and provisions are not toys for us to play with so that we can feel empowered; but tools which we use at His command so that He will be glorified (Philippians 4:19).  That means there is no Divine command which we cannot obey and no gospel mission which we cannot accomplish in His Name and power.  Also, there is no temptation which we cannot overcome (1 Corinthians 10:13).  So, living for Christ is not as impossible as some might think - as long as we really are living for Christ, and not for ourselves.  And His power is not just available in church, but wherever He leads us, including our places of work.